The present invention relates to string digital-to-analog converters (DACs), in particular, to a multi-stage string DAC architecture that may be used in single channel and multi-channel systems.
DACs convert a digital word into a corresponding analog signal. Dual string DACs, a type of DAC, are oftentimes used because of their higher resolution properties that increase dynamic range while reducing truncation noise. FIG. 1(a) shows a simplified block diagram of a conventional dual string DAC 100. The dual string DAC 100 includes two DACs, an MSB String DAC 120 and an LSB String DAC 130, which both include series-connected impedance strings, such as resistor strings. The MSB String DAC 120 typically converts the most significant bits (MSBs) of the digital word, and its output is coupled to the LSB String DAC 130, which converts the least significant bits (LSBs) of the digital word. VOUT represents the converted analog signal.
FIG. 1(b) shows a circuit implementation of the conventional dual string DAC 100. As shown, each string DAC 120, 130 include a resistor string 121.1-121.N, 131.1-131.M coupled to respective switch sets 122.0-122.N, 132.0-132.M that are operated according to digital word based control signals, C0-CN and D0-DM. Also, the switches in switch sets 122.0-122.N, 132.0-132.M generally include only two states—on or off.
The dual string DAC 100, however, is a single channel DAC. To convert a conventional dual string DAC to a multi-channel system, multiple, separate DAC blocks are provided. Alternatively, the MSB DAC can be shared by multiple sub LSB DACs, but buffers are generally needed in between the MSB DAC and each sub LSB DAC to counter loading effects. Typically, two buffers are need per sub-string (i.e., per channel).
The buffers though limit the common-mode range of operation. Also, buffers limit rail-to-rail operation and add extra noise into the conversion. In addition, buffers (or amplifiers) are sources for errors such as nonlinearities due to offset voltages while also taking up large amounts of circuit board space. As such, the use of buffers oftentimes provides more drawbacks than advantages.
Therefore, the inventor recognized a need in the art for string DAC architecture that provides higher resolution while reducing component count and that also compensates for loading effects without using buffers, which can be used in single channel systems and multi-channel systems.